The present invention relates to a method for the production of organic solvent soluble squaraine dyes. More particularly, this invention relates to an improved method of using a series of reactions and reaction mixtures to provide dihydroperimidine squaraine dyes.
Squaraine dyes are known to possess photoconductive and semi-conductive properties. These features have made then very attractive for various industrial applications such as in electrophotographic photoreceptors, organic solar cells, plasma display panels, and optical recording media, and as photopolymerization sensitizers, infrared radiation absorbing inks or paints, and acutance or antihalation dyes in photosensitive media including photothermo-graphic materials.
An early synthetic method for preparing dihydroperimidine squaraine dyes is described by Bello et al., J. Chem. Soc., Chem. Commun., 1993, pp. 452-454. This method is used in EP-A-0 748,465B 1 [counterpart to WO95/23357 and U.S. Pat. No. 5,380,635 (Gomez, et al.)] for providing a squaraine dye having hexanoic acid ester substituents on the outer dihydroperimidine rings. In this method, 1,8-diaminonaphthalene and 1,3-dihydroxyacetone dimer are first reacted to form 2,2-bis(hydroxymethyl)-2,3-dihydroperimidine (the first intermediate). In the second step, the 2,2-bis(hydroxymethyl)-2,3-dihydroperimidine is reacted with squaric acid to form a bis[2,2-bis(dihydroxymethyl) dihydroperimidine] squaraine dye (the second intermediate, a tetrahydroxy squaraine compound). Finally, this tetrahydroxy squaraine intermediate is reacted with hexanoyl chloride to form the dye. U.S. Pat. No. 5,625,062 (Mader et al.) describes a number of other known synthetic methods for preparing squaraine dyes. Each of these known methods is said to suffer from various problems involving reactants, unwanted by-products, low yields, and product instability. Those problems were addressed by the synthesis described in Columns 4 and 5 therein. The synthesis described is similar to that of U.S. Pat. No. 5,380,635 (noted above). The reaction solvents and conditions were changed to improve yields.
One important feature of both U.S. Pat. No. 5,380,635 and U.S. Pat. No. 5,625,062 is the use of the expensive reagent squaric acid in the second reaction step.
U.S. Pat. No. 5,959,105 (Harada et al.) describes another method for preparing squaraine dyes having hexanoic acid ester substituents on the dihydroperimidine rings. Dipentylcarbonyloxymethyl ketone is reacted with 1,8-diaminonaphthalene to form 2,2-dipentylcarbonyloxylmethyl-2,3-dihydroperimidine. Reaction of this diester with squaric acid in a solvent mixture of butanol/toluene forms the squaraine dye having hexanoic acid ester substituents on the dihydroperimidine rings. Purification was said to be achieved by column chromatography on silica gel and elution with chloroform.
Problem To Be Solved
The synthetic methods described above have a number of disadvantages.
In both EP-A-0 748,465B 1 and U.S. Pat. No. 5,625,062 the first reaction step, preparation of 2,2-bis(hydroxymethyl)-2,3-dihydroperimidine intermediate, is carried out in an alcoholic solvent, with or without added acid catalyst. While this procedure can yield good quality product, the alcohol solvent should be completely removed before using the 2,2-bis(hydroxymethyl)-2,3-dihydroperimidine in the subsequent step. A method for preparing this intermediate while avoiding this problem would be desirable.
The second reaction in this synthesis also presents several difficulties. While the second intermediate (tetrahydroxy squaraine) can be obtained in high nominal yield, it is not obtainable in high purity (63-87% described in U.S. Pat. No. 5,625,062). No teaching is provided to purify this intermediate, and its low solubility in most solvents precludes its ready purification by such conventional methods as recrystallization or chromatography. Thus, the impurities must be carried over into the third reaction step, potentially introducing contaminants into the final reaction step.
Moreover, the second reaction step is carried out in either n-propanol (EP-A-0 748,465B1) or a mixtures of solvents including octanol or heptanol (U.S. Pat. No. 5,625,062). The use of n-propanol is undesirable because the second intermediate is difficult to separate from it. The use of mixtures of longer chain alcohols is also undesirable because of their high cost when compared with that of shorter-chain alcohols.
The thermal instability of the second intermediate is also a serious concern from a safety point of view, especially for its large-scale production. To avoid this problem, U.S. Pat. No. 5,625,062 teaches washing the second intermediate with ethanol followed by ethyl acetate. The still-damp second intermediate is then used in the subsequent step. This also introduces several practical and costly problems for large-scale production operations by exposing operators to flammable and toxic solvents or by requiring expensive safety measures.
In the third step of the synthetic method, formation of the squaraine dye, additional difficulties arise. In EP-A-0 784,465B1, the esterification is preformed in the presence of 4-dimethylaminopyridine, an expensive and toxic reagent. In U.S. Pat. No. 5,625,062 the esterification is performed in the presence of pyridine, which while less expensive, is also toxic. Additionally, because of the low solubility of the tetrahydroxy squaraine (second intermediate) in the reaction mixture, it is usually present in the final isolated reaction product (up to 11% is noted in U.S. Pat. No. 5,625,062). This impurity diminishes the usefulness of the desired squaraine dye. The amount of insoluble intermediate remaining varies from batch to batch and requires either an additional filtration step for its removal or results in clogging of coating equipment and poor coatings containing particulates and/or exhibiting streaks.
Still another disadvantage to the synthetic methods noted above relates to the use of squaric acid in the second reaction step. Squaric acid is the most expensive raw material used in preparing squaraine dyes. Due to poor yields in these methods, and the use of squaric acid early in the synthesis, more squaric acid is required to obtain a given quantity of squaraine dye.
Although the synthesis described in U.S. Pat. No. 5,959,105 appears to have only two reaction steps, the dipentylcarbonyloxymethyl ketone also requires preparation. U.S. Pat. No. 5,959,105 provides no teaching as to synthetic methods of its preparation, its yield, or its purity. Moreover, the purification of the final product requires use of the highly toxic solvent chloroform. Using this method for large-scale production would be costly and potentially harmful to the environment.
There is clearly a need for a more efficient method to produce dihydroperimidine squaraine dyes that avoids the difficulties and disadvantages of the known methods described above and that can be readily used in a cost effective manner in large-scale production.
The present invention provides a solution to the problems noted above with a method of making a compound comprising:
A) reacting the following Compound I with the following Compound II to form the following Compound III in a first reaction mixture comprising one or more organic solvents, wherein the molar ratio of Compound I to Compound II initially in the first reaction mixture is from about 1.5:1 to about 2:1, 
B) reacting the resulting Compound III with an organic acylating agent to form the following Compound IV in the presence of one or more organic solvents, wherein the initial molar ratio of Compound III to the organic acylating agent is from about 0.4:1 to about 0.6:1,
wherein R in Compounds IV and V below is a monovalent organic radical derived from the organic acylating agent, 
xe2x80x83and
C) reacting the resulting Compound IV with squaric acid to form the following Compound V in a separate reaction mixture under conditions that provide for at least partial removal of water from the separate reaction mixture, the separate reaction mixture comprising at least one hydroxylic organic solvent and at least one non-polar organic solvent, such that the squaric acid is at least partially soluble in the hydroxylic organic solvent, and the weight ratio of the hydroxylic organic solvent to the nonpolar solvent is sufficient to provide a ternary azeotrope with water, 
wherein the molar ratio of the Compound IV to squaric acid initially in the separate reaction mixture is at least 1.8:1.
In a preferred embodiment of this invention, a method of making a compound comprises:
A) reacting the following Compound I with the following Compound II to form the following Compound III in a first reaction mixture comprising one or more organic solvents comprising predominantly at least one alkyl ester, wherein the molar ratio of Compound I to Compound II initially in the first reaction mixture is from about 1.5:1 to about 2:1, 
B) with or without isolation, reacting the resulting Compound III with an organic acylating agent to form the following Compound IV in a second reaction mixture comprising one or more organic solvents comprising predominantly at least one alkyl ester, wherein the molar ratio of Compound III to the organic acylating agent initially in the second reaction mixture is from about 0.4:1to about 0.6:1,
wherein R in Compounds IV and V below is a monovalent organic radical derived from the organic acylating agent, 
xe2x80x83and
C) reacting the resulting Compound IV with squaric acid to form the following Compound V in a third reaction mixture under conditions that provide for at least partial removal of water from the third reaction mixture, the third reaction mixture comprising at least one hydroxylic organic solvent and at least one non-polar organic solvent, such that the squaric acid is at least partially soluble in the hydroxylic organic solvent, and the weight ratio of the hydroxylic organic solvent to the nonpolar solvent is sufficient to provide a ternary azeotrope with water, 
xe2x80x83wherein the molar ratio of the Compound IV to squaric acid initially in the third reaction mixture is at least 1.9:1.
Several advantages have been achieved with the practice of the present invention compared to the several known methods for making squaraine dyes noted above. First of all, while three reactions are used instead of two (in comparison to U.S. Pat. No. 5,959,105), the reactants and solvents are readily available, inexpensive, generally less toxic and require fewer isolation procedures. The method is readily scaled up for production operations with lessened impact on the environment, reduced hazards to operators and at less expense.
With reference to the three-step methods of U.S. Pat. No. 5,625,062 and other cited references, the method of the present invention uses the expensive squaric acid reactant in the last step, thereby maximizing its incorporation into the desired final product. Required isolation techniques are reduced, and can be even avoided entirely in some embodiments of the method. For example, in embodiments wherein the same solvents such as alkyl esters are used in Steps A and B, there may be no need to isolate Compound m, the first intermediate formed in Step A before Step B can be carried out. Thus, the first and second reactions (in Steps A and B) defined herein may be carried out in the same reaction vessel having all necessary components (Compounds I and II, and organic acylating agent) present at the beginning of the method in solvents that can be used during both reactions. Alternatively, the reactants and/or different solvents for the second reaction can be added some time after the first reaction has begun, and these additional reactants and/or solvents can be added all at once, in portions or in a continual stream.
In addition, since no tetrahydroxy squaraine intermediate is produced in the second reaction of the method of this invention, fewer insoluble compounds are produced that have to be removed or carried over into the final product mixture.
All of the various features of the three reactions combine to provide a highly efficient method of making the desired squaraine dyes at increased yields and higher purity.
Lastly, the method of this invention allows the preparation of squaraine dyes having increased solubility in polar solvents, such as methyl ethyl ketone, that may be useful as coating solvents for various uses of the dyes. This reduces the potential for coating defects and coating equipment clogging from the presence of insoluble particulate materials.
The method of this invention has utility to prepare the noted squaraine dyes. Those dyes have several industrial utilities, some of which are pointed out above in the first paragraph of the Background of the Invention. A preferred utility is to use the dyes as acutance or antihalation dyes in photothermo-graphic materials, for example as described in U.S. Pat. No. 5,380,635 (noted above), and U.S. Pat. No. 6,063,560 (Suzuki et al.), both incorporated herein by reference for details regarding this preferred utility.
The method of this invention generally involves the reaction of 1,8-diaminonaphthalene with 1,3-dihydroxyacetone dimer in a first reaction mixture comprising the appropriate solvents to prepare a first intermediate. This first intermediate is then reacted with an organic acylating agent in suitable solvents to form a second intermediate. Lastly, the second intermediate is reacted with squaric acid in a particular hydroxylic/nonpolar organic solvent mixture that dissolves squaric acid and forms a ternary azeotrope with water so that at least some of the water is removed during the reaction. The details of the method of this invention will be presented now in reference to the three reactions followed by discussion of optional features of the method and the working examples.
First Reaction:
In a first reaction, Compound I (1,8-diaminonaphthalene) is reacted with Compound II (1,3-dihydroxyacetone dimer) to obtain Compound III [2,2-bis(hydroxymethyl)-2,3-dihydroperimidine] that is identified herein as the xe2x80x9cfirst intermediatexe2x80x9d. Compounds I and II are commercially available for example from Aldrich Chemical Company, Milwaukee, Wis.
This reaction is carried out in a first reaction mixture that includes one or more organic solvents that dissolve the reactants and from which the first intermediate can be readily isolated if necessary. In preferred embodiments, the organic solvents include predominantly at least one alkyl ester. By xe2x80x9cpredominantlyxe2x80x9d is meant that the one or more alkyl esters comprise at least 50 weight % of the weight of total solvents. Preferably, the one or more alkyl esters comprise at least 80 weight %, and more preferably at least 95 weight %, of the weight of total solvents. In the best practice of this invention, the only solvent in the first reaction mixture is an alkyl ester.
Particularly useful alkyl ester solvents include but are not limited to, alkyl acetates, alkyl propionates, and alkyl butyrates. Alkyl acetates, such as ethyl acetate, n-propyl acetate, n-butyl acetate, isopropyl acetate, and isobutyl acetate are preferred. Mixtures of these are also useful. A most preferred solvent is n-propyl acetate.
Besides alkyl esters, other organic solvents that may be used in the first reaction mixture include alcohols (such as ethanol, isopropanol, n-propanol, and n-butanol), toluene and heptane, and mixtures of any of these.
The first reaction is generally carried out until a sufficient amount of Compound III is obtained. Generally, this requires at least 60 minutes, and preferably from about 60 to about 180 minutes. Reaction temperature can be varied depending upon the solvents and the desired reaction time. Generally, the reaction temperature is at least 50xc2x0 C., and preferably from about 50 to about 150xc2x0 C., and more preferably from about 90 to about 101xc2x0 C. A skilled worker would be able to determine the optimum reaction conditions for given solvents and desired yields.
The molar ratio of Compound I to Compound II initially in the first reaction mixture is from about 1.5:1 to about 2:1. Preferably, this molar ratio is from about 1.9:1 to about 2:1.
The first reaction may also include optional features and conditions that enhance completion of the reaction, safety or other desired results, including suitable agitation, gradual removal of solvent and water by distillation and reaction under an inert atmosphere such as nitrogen or argon.
Second Reaction:
Compound III [2,2-bis(hydroxymethyl)-2,3-dihydroperimidine] formed from the first reaction is reacted with an organic acylating agent to form Compound IV that is identified herein as the xe2x80x9csecond intermediatexe2x80x9d.
A variety of acylating agents and a variety of acylating methods can be used in the second reaction. Non-limiting examples of these agents and methods include:
a) reaction with an acid halide or an acid anhydride in the presence of a base such as an organic base (for example, an amine such as triethyl amine), a metal salt (for example, sodium acetate, sodium bicarbonate, or potassium carbonate), or a metal hydroxide (for example sodium or potassium hydroxide),
b) reaction with a carboxylic acid in the presence of an acid catalyst (for example, sulfuric acid, toluenesulfonic acid, or hydrogen chloride) usually with the removal of water (using for example, an azeotrope, molecular sieves, or an anhydrous salt),
c) reaction with a carboxylic acid in the presence of a dehydrating agent (for example, a carbodiimide such as dicyclohexylcarbodiimide or diisopropylcarbodiimide), N,Nxe2x80x2-carbonyldiimidazole, trifluoroacetic anhydride, or triphenyl phosphine with a dialkyl azodicarboxylate, and
d) reaction by transesterification using a carboxylic ester (for example, methyl propionate, ethyl butyrate, or methyl hexanoate) in the presence of an acidic catalyst (for example p-toluenesulfonic acid) or a Lewis acid (such as titanium tetramethoxide) or a basic catalyst (such as sodium methoxide or potassium hydroxide).
These and additional methods can be found in xe2x80x9cAdvanced Organic Chemistryxe2x80x9d, Jerry March, 2nd Ed., McGraw-Hill Book Co., New York, USA, 1977, pages 361-367; and in xe2x80x9cComprehensive Organic Functional Group Transformationsxe2x80x9d, Volume 5, J. Mulzer; [A. R. Katritzky, O. Meth-Cohn, and C. W. Rees, Eds.], Elsevier Science Ltd., Oxford, UK, 1995, pages 122-139.
While mixtures of organic acylating agents can be used in the second reaction, the result would be a mixture of second intermediates that may or may not be useful in the third reaction, and may require additional isolation steps. Preferably, only a single organic acylating agent is used.
In a preferred embodiment, the organic acylating agent is an acid halide or an acid anhydride that is used in the presence of an amine. Useful organic acid halides can be any organic compound having an acid halide moiety and a suitable aliphatic or aromatic moiety that provides the xe2x80x9cRxe2x80x9d group in Compounds IV and V. Useful classes of organic acid halides and anhydrides include but are not limited to, aliphatic acid halides, carbocyclic aromatic acid halides, and heterocyclic acid halides. Thus, R can be any suitable substituted or unsubstituted alkyl group (branched or linear) having 1 to about 15 carbon atoms, substituted or unsubstituted cycloalkyl group having 5 to 10 carbon atoms in the cyclic system, substituted or unsubstituted aryl groups having 6 to 10 carbon atoms in the aromatic system, substituted or unsubstituted heterocyclic groups having 5 to 10 atoms in the cyclic system (including carbon, nitrogen, sulfur and oxygen atoms). Preferably, R is a substituted or unsubstituted alkyl group having 3 to 7 carbon atoms (all possible isomers).
Useful organic acid halides are chlorides, bromides, and iodides of organic acids. Useful organic acid halides include but are not limited to, acid chlorides, bromides, and iodides of pentanoic acid, hexanoic acid, heptanoic acid, octanoic acid, benzoic acid, butyric acid, and phenylacetic acid. Useful organic acid anhydrides include butyric anhydride, valeric anhydride, and hexanoic anhydride.
Aliphatic acid chlorides are preferred. Preferred acid chlorides are aliphatic acid chlorides with substituted or unsubstituted alkyl group having 3 to 7 carbon atoms. A most preferred acid chloride is n-hexanoic acid chloride.
In a preferred embodiment, the second reaction is carried out in the presence of one or more amines that catalyze the reaction and neutralize the hydrogen halide formed. Useful amines include aliphatic amines and cyclic amines. The aliphatic amines are preferred. Examples of useful amines include but are not limited to, triethylamine, tri-n-propylamine, tri-n-butylamine, pyridine, diisopropylethyl amine, N-methyl-piperdine, and N-methyl-morpholine. A most preferred amine is triethylamine.
In some embodiments, the second reaction is carried out in a distinct second reaction mixture that includes one or more organic solvents that dissolve the reactants and from which the second intermediate can be readily isolated if necessary. In such embodiments, the organic solvents preferably include predominantly at least one alkyl ester as defined for the first reaction mixture. Thus, preferably the one or more alkyl esters comprise at least 50 weight % of the weight of total solvents. More preferably, the one or more alkyl esters comprise at least 80 weight %, and most preferably at least 95 weight %, of the weight of total solvents. In the best practice of this invention, the only organic solvent in the second reaction mixture is an alkyl ester.
Particularly useful alkyl ester solvents include but are not limited to, alkyl acetates, alkyl propionates, and alkyl butyrates. Alkyl acetates, such as ethyl acetate, n-propyl acetate, n-butyl acetate, isopropyl acetate, and isobutyl acetate are preferred. Mixtures of these are also useful. A most preferred solvent is n-propyl acetate.
Besides alkyl esters, other organic solvents that may be used in the second reaction include nonhydroxylic solvents such as tetrahydrofuran (THF), toluene, dimethoxyethane, methylene chloride, and mixtures of any of these.
Alternatively, as pointed out above, the second reaction may be carried out by adding or including the components necessary for the second reaction to the first reaction mixture. Thus, the first and second reactions can be carried out sequentially or simultaneously, or even in stages (for example, first reaction, second reaction, first reaction and second reaction) by controlling addition of necessary reactants. In such embodiments, the solvents used must be compatible with both reactions, and preferably, they are the same solvents.
Irrespective of how the second reaction is carried out in relation to the first reaction, the second reaction is generally carried out until a sufficient amount of Compound IV is obtained. Generally, this requires at least 60 minutes, and preferably from about 60 to about 180 minutes. Reaction temperature can be varied depending upon the solvents and the desired reaction time. Generally, the reaction temperature is at least 0xc2x0 C., and preferably from about 0 to about 50xc2x0 C., and more preferably from about 5 to about 30xc2x0 C. A skilled worker would be able to determine the optimum reaction conditions for given solvents and desired yields.
The molar ratio of Compound III to the organic acylating agent initially used to carry out the second reaction is from about 0.4:1 to about 0.6:1. Preferably, this molar ratio is from about 0.45:1 to about 0.50:1.
The second reaction may also include optional features and conditions that enhance completion of the reaction, safety or other desired results, including suitable agitation, and an inert atmosphere, such as nitrogen or argon.
In all preferred embodiments, the organic solvents used in the first and second reaction mixtures are the same, and most preferably, those organic solvents are alkyl esters such as ethyl acetate, n-propyl acetate, n-butyl acetate. Preferably, Compound IV is isolated from the reaction mixture (either first or second depending upon how the second reaction is carried out) before the third reaction is carried out using any suitable procedure.
Third Reaction:
Compound IV (a diester dihydroperimidine) is reacted with squaric acid to form the desired squaraine dye represented by Compound V. Squaric acid is commercially available from a number of sources such as Aldrich Chemical Company, Milwaukee, Wis.
This reaction is carried out in a separate (usually third) reaction mixture that includes one or more polar hydroxylic organic solvents and one or more nonpolar organic solvents. At least one hydroxylic organic solvent is used to dissolve squaric acid, and the hydroxylic and nonpolar organic solvents are present at a weight ratio sufficient to provide a ternary azeotrope with water. One skilled in the art would be able to readily determine the suitable weight ratio for given solvents knowing their boiling points and composition of the ternary azeotrope. Preferably, the weight ratio of nonpolar organic solvent to hydroxylic organic solvent is from about 1:2 to about 3:1, and preferably from about 1:1 to about 2:1.
By xe2x80x9chydroxylicxe2x80x9d is meant that the solvent comprises one or more hydroxy groups. Useful hydroxylic organic solvents include but are not limited to, alcohols having from 3 to 7 carbon atoms (various isomers or mixtures of isomers), 2-ethoxyethanol, and 2-methoxyethanol. For purposes of this invention, hydroxylic solvents are include solvents that include one or more hydroxy groups as part of one or more carboxy groups. Such solvents include but are not limited to, acetic acid. The noted alcohols are preferred, and n-butanol, n-propanol, and n-pentanol are more preferred.
Useful nonpolar organic solvents include but are not limited to, hydrocarbons having 6 to 10 carbon atoms (all possible isomers or mixtures of isomers), chlorobenzene, and toluene. The noted hydrocarbons are preferred, and hexanes, heptanes, and cyclohexane are more preferred.
Representative organic solvent mixtures that can be used in the third reaction include but are not limited to, n-butanol/n-heptane, n-butanol/toluene, and n-butanol/cyclohexane. A mixture of n-butanol and n-heptane is most preferred.
The third reaction is carried out until a sufficient amount of Compound V is obtained, in high yield and purity. Generally, this requires at least 60 minutes, and preferably from about 60 to about 180 minutes. Reaction temperature can be varied depending upon the solvents and the desired reaction time. Generally, the reaction temperature is at the boiling point of the solvent mixture, and for preferred solvents, this means from about 80 to about 100xc2x0 C. A skilled worker would be able to determine the optimum reaction conditions for given solvents and desired yields. Overall, the conditions and organic solvents are chosen so that water is at least partially, and preferably completely, removed during the reaction using conventional techniques.
The molar ratio of Compound IV to squaric acid initially in the third reaction mixture is at least 1.8:1. Preferably, this molar ratio is from about 2:1 to about 2.2:1.
The third reaction may also include optional features and conditions that enhance completion of the reaction, safety or other desired results, including suitable agitation, gradual removal of solvents and water by distillation for example, reaction in an inert atmosphere such as nitrogen or argon. Preferably, the desired squaraine dyes are isolated from the third reaction mixture using any suitable procedure such as filtration.
As would be understood by one skilled in the art, the various reaction mixtures may be heated or cooled between steps or during individual reactions to encourage the reactions to completion or to manage any exotherms. Any suitable heating and cooling means can be used. The reaction vessels to be used for each reaction are not critical although glass-lined vessels may be preferred.
In a preferred embodiment, the predominant organic solvent in both the first and second reactions is n-propyl acetate. In the second reaction, the preferred amine is triethylamine and the preferred acylating agent is a hexanoyl acid chloride (for example, n-hexanoic acid chloride). For the third reaction, the polar organic solvent is n-butanol and the nonpolar organic solvent is n-heptane.
Moreover, in preferred embodiments, the reaction conditions are as follows:
The first reaction is carried out at a temperature of from about 90 to about 101xc2x0 C. for from about 60 to about 180 minutes, and Compound III can be isolated by cooling and filtration,
The second reaction is carried out at a temperature of from about 5 to about 30xc2x0 C. for a time of from about 60 to about 180 minutes, and Compound IV can be isolated by aqueous washing, removing solvents, adding methanol, cooling, filtering, and drying.
The third reaction is carried out at a temperature of from about 80 to about 100xc2x0 C. for a time of from about 60 to about 180 minutes, and Compound V is isolated from the separate or third reaction mixture by cooling and filtration.
The preferred method of this invention, used to prepare a preferred squaraine dye, cyclobutenedilyium, 1,3-bis[2,3-dihydro-2,2-bis[[1-oxohexyl)oxy]-methyl]-1H-perimidin-4-yl]-2,4-dihydroxy-, bis(inner salt), is shown in the following Reaction Scheme: 